Ultraminiature pressure sensor and method of making same

ABSTRACT

A capacitive pressure sensor suitable for making highly sensitive, low pressure measurements is disclosed. The sensor may be mounted into a 0.5 mm OD catheter suitable for multipoint pressure measurements from within the coronary artery of the heart. The sensor employs a transducer which consists of a rectangular bulk silicon micro-diaphragm several hundred microns on a side by two microns thick, surrounded by a supporting bulk silicon rim about 12 microns thick. Both the diaphragm and the rim are defined by a double diffusion etch-stop technique. The transducer fabrication process features a batch wafer-to-glass electrostatic seal followed by a silicon etch, which eliminates handling of individual small diaphragm structures until die separation and final packaging. An addressable read-out interface circuit may be used with the sensor to provide a high-level output signal, and allows the sensor to be compatible for use on a multisite catheter having only two electrical leads.

This application is a division of U.S. patent application Ser. No.057,884 filed June 1, 1987 now U.S. Pat. No 4,815,472.

FIELD OF THE INVENTION

This invention relates in general to solid-state pressure sensors andmethods of making them, and in particular to silicon pressure sensorshaving a diaphragm and supporting rim structure made frommonocrystalline silicon wafers processed using etch-stop techniques.

BACKGROUND OF THE INVENTION

There are many applications requiring the accurate measurement ofpressure, ranging from the monitoring of physiological parameters formedical research, to the precise control of fluids or gases, tomeasurement of low energy acoustical signals. Typical applicationsinclude industrial process monitoring, such as the monitoring of gasflow under partial vacuums in semiconductor processing facilities to theprecise control of air/fuel ratios in automobiles. Typical medicalapplications include measurement of blood pressure in surgery and inintensive care, air pressure in respiratory diseases, intrauterinepressure in obstetrics, abdominal and urinary pressure for diagnosis ofdisorders, and the like. In some such applications, it is desirable tomeasure pressure with an extremely small sensor so as not to disturb thesystem being monitored. For example, cardiovascular catheterization hasbecome a major and common diagnositic tool in dealing with thecardiovascular system. In angioplasty (balloon pumping) to treatocclusions in the coronary artery of the heart, there is presently nosatisfactory means of judging the results on-line, that is, as treatmentis being administered. Existing catheter-tip pressure sensor aresingle-point, not highly reliable, very expensive, and too large for usewithin the coronary artery. Also, they typically offer only low-leveloutput signals which are very susceptible to noise and artifact.

Recent advances in silicon micromachining technology have allowed todevelopment of a wide range of solid-state pressure sensor. Amongst themost useful, on account of their increased sensitivity, are capacitivepressure sensors. In the past few years the use of impurity-sensitiveetch-stops and deposited diaphragm structures have resulted in precise,ultrathin diaphragms that have substantially broadened the range ofpractical structures which can be realized. In particular, a significantminiaturization of solid-state pressure sensors is now feasible. See forexample, H. Guckel, et al, "Laser-Recrystallized PiezoresistiveMicro-Diaphragm Sensor," Dig. Tech. Papers, IEEE Int. Conf. Solid-StateSensors and Actuators, pp. 182-185 (June 1985).; R. S. Hijab and R. S.Muller, "Micro-mechanical Thin-Film Cavity Structures for Low Pressureand Acoustic Transducer Applications," Dig. Tech. Papers, IEEE Int.Conf. Solid-State Sensors and Actuators, pp. 178-181 (June 1985).; andS. Sugiyama, et al, "Micro-Diaphragm Pressure Sensor," IEDM Tech. Dig.,December 1986. For a general review of factors affecting performance anddown-sizing of pressure sensor, see H. L. Chau and K. D. Wise, "ScalingLimits in Batch-Fabricated Silicon Pressure Sensors," IEEE Trans.Electron Devices, Vol. ED-34, April 1987. Thus, some techniques are nowknown for fabricating thin diaphragms essential for the ultraminiaturesensors. However, improved fabrication techniques are needed along withimproved sensor packaging and interface electronics in order to allowthe sensors to be scaled downwardly while maintaining high performanceand high yield. The most commonly used techniques for forming adiaphragm and supporting rim structure from a silicon wafer involveanisotropically etching a recess for the reference cavity of thetransducer on the front side a silicon wafer, and selectively etchingaway 90% or more of the back of the silicon wafer in order to form adiaphragm of desired thickness. The diaphragm thickness is typicallycontrolled using a boron buried layer or a p-n junction etch-stop. Thisback etching is done normally with preferential etchants which give abevel having a wall angle of about 52 degrees. Due to the thickness ofthe silicon wafer, much lateral area around the diaphragm of thetransducer is required, thus making it difficult to produce a smalldevice. This use of lateral space is not productive, in that it is not afunctional part of the sensor.

Many solid-state pressure tranducers or sensors are made using a siliconwafer which is first preferably etched, and then electrostaticallybonded to a glass substrate. In many such sensors, particularly thosehaving larger diaphragms, the electrostatic bonding process requires theapplication of a very high applied field which tends to pull thediaphragm over the glass and weld it to the glass, thus resulting in aninoperable device. To avoid this problem, a field shield plate, whichcan be one of the electrodes of the capacitive transducer, is groundedduring the sealing processing. However, grounding of individualcapacitor plates during a batch process is very hard to arrange, and ismore effectively done only when the transducers are bonded to asupporting piece of glass one at a time. However, to produce commercialquantities of transducers, it is highly desirable to be able to seal anentire wafer of silicon transducers to a glass plate at one time withouthaving to ground the individual capacitor plates of the transducers.

Another problem with many techniques used for producing solid-statepressure sensors is that numerous processing steps are required,including a number of steps requiring critical alignment and masks andthe like, thus increasing costs and reducing yield. Thus, it woulddesirable to provide for a simpler, more reliable process requiringfewer processing steps and fewer critical alignment steps. In thisregard, it is noted that existing techniques for making ultraminiaturepressure sensors are typically costly. For example for sensorsapproaching one millimeter in diameter, devices may cost as much asseveral hundred dollars each. Clearly, it would very desirable toprovide a pressure sensor structure and method of making it which wouldpermit the cost of producing such ultraminiature devices to be reducedconsiderably, perhaps by as much as an order of magnitude or more.

In light of the foregoing, it is a primary object of the presentinvention to provide an improved solid-state pressure sensor structureand method of making it which allows ultraminiature pressure sensors tobe fabricated with fewer, less costly steps and greater yield. Otherobjects of the present invention include eliminating the large rim areasassociated with existing pressure sensors having a diaphragm and rimstructure made from bulk silicon, and eliminating the need to provide afield shield plate during the electrostatic bonding process.

Other objects of the present invention include providing a pressuresensor that is capable of multipoint operation, is addressable, and iscompatible for use on a multisite catheter having only two leads, namelythe electrical power supply leads. One more object is to provide such asensor which allows on-chip temperature measurement for purposes ofcompensation. Yet another object is to provide such a catheter systemsuitable for medical uses such as cardiovascular catheterization.Additional objects of the invention include providing a transducerfabrication process that is fully batch in nature and does not requireindividual handling of small parts.

SUMMARY OF THE INVENTION

In light of the foregoing problems and to fulfill one or more of theforegoing objects, the present invention provides an ultraminiaturecapacitive pressure sensor having a silicon diaphragm and rim structuremade with a simple double-diffusion process. This novel diaphragm andrim structure is part of a silicon transducer chip which iselectrostatically bonded to a glass support plate prior to removal ofall of the wafer except for the diaphragm and rim structure. The noveldiaphragm and rim structure features a very small rim area, thusallowing the transducer to be constructed in ultraminiature form. Thus,capacitive pressure sensor of the present invention can be mounted, forexample, in a 0.5 millimeter OD multisite cardiac catheter suitable formeasuring blood pressure gradients inside the coronary artery of theheart. The silicon pressure transducer preferably includes supportinginterface circuitry on a chip fastened to the same glass support plateas the diaphragm and rim structure.

According to one aspect of the invention, there is provided a method ofmaking a pressure sensor having a diaphragm in rim structure includingbulk silicon, which comprising the steps: (a) providing a silicon wafer;(b) forming at least one mesa upon the silicon wafer to be used as partof the rim structure of the pressure sensor; (c) impregnating a selectedportion of the silicon wafer which includes the mesa with at least afirst material which alters an etching characteristic of the firstselected portion; (d) impregnating a second selected portion of thesilicon wafer which will become the diaphragm of the sensor with asecond material which alters an etching characteristic of the secondselected portion; and (e) removing by etching at least a selected thirdportion of the silicon wafer adjacent to the first and second portionsas part of forming the diaphragm and rim structure. Steps (c) and (d)are preferably performed by a deep diffusion and a shallow diffusionrespectively of an impurity dopant, namely boron, into the siliconwafer.

According to a second aspect of the invention, there is provided anultraminiature solid-state capacitive pressure transducer, comprising:an integrally formed structure made from single-crystal material andhaving a diaphragm and a rim extending about a substantial portion ofthe periphery of the diaphragm, the structure having at least twodimensions orthogonal to one another of less than one millimeter. Thetransducer typically includes a glass plate that is electrostaticallybonded to at least part of the rim structure, and the single-crystalmaterial is typically a silicon. The rim and diaphragm and heavilyimpregnated with at least a first material which alter an etchingcharacteristic of the rim and diaphragm in comparison to single-crystalmaterial which contains substantially less of such first material. Thefirst material is typically a impurity dopant selected from a group ofdopants including n-type materials and p-type materials, with thepreferred p-type material being boron.

According to a third aspect of the present invention, there is provideda multipoint pressure-measuring catheter system, comprising: a catheter;a plurality of pressure sensors spaced along the catheter; and singleconduit means within the catheter for providing a path for signals to bepassed between an external monitor and each of the pressure sensors. Thepressure sensors each include pressure transducer means for converting asensed pressure into an internal signal, switching means for applyingthe internal signal to the signal conduit means, and addressing meansresponsive to a command signal from the external monitor for selectivelyinterrupting switching means, whereby the external monitor may receiveseparately the internal signal generated by each of the pressuresensors.

These and other aspects, objects and advantages of the present inventionwill be better understood by reading the following detailed descriptionin conjunction with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings form an integral part of the description of the preferredembodiments and are to read in conjunction therewith. Like referencenumerals designate components in the different Figures, where:

FIG. 1 is an exploded perspective view of an ultraminiature capacitivepressure sensor of the present invention which includes a siliconpressure transducer and integrated circuit chip mounted on a glasssubstrate;

FIGS. 2A-2H are a series of cross-sectional side views taken along ine2--2 of FIG. 1 showing the various processing steps associated withfabricating the silicon transducer from a silicon wafer, and bonding itto the glass substrate;

FIG. 3 is a top cross-sectional view of the silicon pressure transducerof FIG. 1 after it is bonded to the glass substrate;

FIG. 4 is a plan view showing several completed glass silicontransducers from a matrix array of such chips which have beensimultaneously bonded to a glass plate using the batch processing stepsof FIG. 2, prior to dicing the plate into individual sensors;

FIG. 5 is a graph showing the capacitance change versus applied pressurecharacteristics of one ultraminiature pressure sensor constructed inaccordance with the present invention;

FIG. 6 is a functional block diagram of a preferred embodiment for theon-chip circuitry used in the FIG. 1 pressure sensor;

FIG. 7 is a circuit diagram of the pulse amplitude discriminator moduleof FIG. 6;

FIG. 8 is a circuit diagram of the two-stage counter of FIG. 6;

FIG. 9 is a detailed block diagram of the Schmitt trigger oscillatormodule of FIG. 6;

FIG. 10 is a signal timing diagram showing waveforms and timingrelationships of various signals in the circuitry illustrated in FIGS.6-10; and

FIG. 11 is a multipoint pressure-sensing catheter system of the presentinvention which utilizes two of the FIG. 1 pressure sensors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an ultraminiature pressure sensor 30 of the presentinvention is shown in an exploded perspective view for ease ofunderstanding. The pressure sensor 30 is comprised of three maincomponents: a patterned glass support substrate 32 selectivelymetallized in certain areas, a patterned silicon transducer chip 34, andan interface circuit chip 36. The glass substrate has formed two thereingrooves 40 and 42 at the left end 44 of the glass substrate 32, and asecond pair of grooves 46 and 48 at the right end 50 of the glasssubstrate. Formed just beyond the inner end of the groove 40 and 42 aremetallized bonding pads 60 and 62. Similarly, just beyond the inner endsof grooves 46 and 48 are metallized bonding pads 66 and 68. Typically,the grooves have a trapezoidal cross-section. The grooves 40, 42, 46 and48 are metallized and are electrically connected respectively to pads60, 62, 66 and 68. The grooves 40 and 42 receive wires 70 and 72, whilegrooves 46 and 48 receive wires 76 and 78. The wires 80, 82, 86 and 88are respectively soldered or otherwise secured to grooves 40, 42, 46 and48 prior to using the pressure sensor 30, as will later be described.

Upon the top surface 38 of the glass substrate 32 is a patternedmetallized area or region 90 containing a first (rear) and second(front) interconnect traces 96 and 98, which follow the periphery of andare spaced from a metallized rectangular pad 100 which serves asone-half of the active capacitor C_(X) of the pressure transducer ofsensor 30 namely the lower plate of capacitor C_(X). In region 90 arethree metallized pads 106, 107 and 108 spaced apart from one another ina line perpendicular to the central longitudinal axis 105 of the glasssupport substrate 32. The first interconnect trace 96 is connected atone end thereof to pad 106, and at the other end thereof to pad 66. Thesecond trace 98 is connected at one end thereof to pad 108 and at theother end thereof to pad 68. An enlarged view of the traces 96 and 98and capacitor 100 is provided in FIG. 3. The circuit chip 36 contains afirst set of beam leads 110 and 112, at the left and thereof, and asecond set of beam leads 116, 117 and 118 at the right end thereof. Asindicated by dashed lines 120, the circuit chip 36 is assembled to theglass support substrate 32 by flipping chip 36 over so that the beamleads 110, 112, 116, 117 and 118 respectively about against and may bebonded to bonding pads 60, 62, 106, 107 and 108 by ultrasonic bonding orthermocompression.

The patterned transducer chip 34 is preferably made from a conventionalsingle crystal silicon wafer of the type widely used in thesemiconductor industry. The chip 34 contains a very thin centrallylocated diaphragm 130 which is integrally connected to an surrounded bya much thicker rectangular rim 132. The rim 132 is preferably formedwith at least two reference pressure inlet channels 133 and 134respectively located on opposite sides of the rim, and which (untilsealed off) provide access to a small space or chamber 135 underdiaphragm 130. The silicon transducer chip 34 is electrostaticallybonded to the glass substrate 32 in the location indicated in phantomby-dotted lines 136. The circuit chip 36 preferably contains all of thenecessary read-out electronics, as will be further explained.

Key parts of the structure and process for making the pressure sensor 30are the structure and process for making the transducer chip 34, and theprocess for joining of this chip to the glass substrate 32. This processand the resulting structure is illustrated in FIGS. 1 through 4.

The fabrication process starts with a 250 micron thick borosilicateglass plate 138 which may be a Corning 7740 glass plate, and whichbecomes the patterned glass plate 32 after processing. Since the heightof the gap between the metallized capacitor plate 100 and silicondiaphragm 130 shown in FIG. 1 is typically less than 5 microns, a verysmooth finish on both plates 100 and 140 are important in achieving highyield. Grooves 40, 42, 46 and 48 are first etched into the glass plate32 with concentrated hydrofluoric (HF) acid using a gold/chromium mask.The gold/chromium mask is preferably formed in two layers by vacuumevaporation, with the chromium layer being about 200 to 400 angstromsthick and eing deposited first, and the gold layer being about 3,000 to5,000 angstroms thick. This mask is patterned using conventionalphotolithographic techniques so that the HF acid isotropically attacksthe glass only in the area where the grooves are desired. Once thegrooves are formed, the two layer mask is completely removed and asecond multiple-layer conductive coating is deposited everywhere on theglass, and photolithographically patterned to form the appropriatemetallized areas, namely grooves 40, 42, 46 and 48, bonding pads 60, 62,66 and 68, interconnects 96 and 98 the lower plate 100 of the activecapacitor (i.e., the transducer capacitor), and the metal pads 106, 107and 108. Using batch processes, sites for numerous identical sensors 30are created on a single sheet of glass 138.

The multiple-layer conductive coatoing may be made of a first layer ofchromium about 300 angstroms thick for good adhesion to the glasssubstrate 32, and a second layer of gold about 2000 to 4000 angstromsthick for making good electrical contact with the wires, beam leads andthe like connected thereto. Alternatively, a combination of three metallayers consisting of a first layer of titanium, a second layer ofplatinum, and third or top layer of gold could be used as the conductivecoating. In this alternative conductive coating, the titanium andplatinum layers should have a combined thickness of about 200 to 300angstroms, while the gold layer should have a thickness of 2000 to 4000angstroms. As is shown in FIG. 3, an area of overlap 139 between the rim132 and the interconnect trace 96 in the vicinity of the metal pad 66provides the electrical connection between the rim 132 and trace 96. Therim 132 serves as the conductive path leading to the top plate of thetransducer capacitor, namely the silicon diaphragm 130.

Referring to the various views of FIG. 2, the processing of the silicontransducer chip 34 will now be explained. Starting with either n-type orp-type 100-oriented silicon wafer having a resistivity of about 6-8ohm-centimeters, the wafer 140 is first cleaned. The wafer 140 is nextthermally oxidized to a thickness of about 0.5 microns on all surfacesas indicated by a top layer 142 and bottom layer 144 of silicon dioxidein FIG. 2A. The wafer 140 is then coated with a layer of photoresist,which is then patterned in accordance with a first mask so as to leaveresist in two flattened U sections 146 and 148 shown in FIG. 3, withcross-sectional portions of sections 146 and 148 thereof being shown inFIG. 2A. The wafers are aligned in a mask aligner so that the110-orientation flat is parallel to the long side of the diaphragm 100.The silicon dioxide of top layer 142 is then patterned, and undesiredareas thereof removed, leaving SiO₂ portions 152 and 154 located underthe remaining photoresist portions 146 and 148. The photoresist portion146 and 148 are then removed.

Next as shown in FIG. 2B, the shallow recessed sections 156, 158 and 160are removed by etching the top of the silicon wafer 140 using potassiumhydroxide as an etchant and the silicon dioxide portions 152 and 154 asa mask. The recesses 156-160 have positioned therebetween unetched mesas162 and 164 respectively located under silicon dioxide portions 152 and154. The recesses 156 and 160 provide the horizontal spacing betweenadjacent transducers, while the recess 158 forms the gap which becomespace 135 in the transducer capacitor. Next, the silicon dioxideportions 152 and 154 are etched away with buffered HF acid. Thereafter,a new layer of silicon dioxide is thermally grown to a thickness ofabout 1.2 microns and conforms to the existing mesa structure shape. Itis then photolithography patterned in accordance with a second maskwhose geometry is depicted in FIG. 3 by large and small rectangles 176and 178 shown in dashed lines. After this patterning, as shown in FIG.2C, the silicon dioxide portions 178, 180 and 181 remain.

Referring again to FIG. 2C, the silicon wafer 140 is then impregnated inselected areas with etch-rate altering impurities by performing a deepboron diffusion, using the oxide mask portions 178, 180 and 181 toprevent the dopant gas from diffusing too far thereunder. This deepboron diffusion step defines the intended rim areas, such as rimportions 182 and 184. The deep boron diffusion is preferably performedat 1175 degrees C. for 15 hours using a solid dopant source (e.b., boronnitride) to provide a desired rim thickness as will be furtherexplained. The doped wafer is then placed in a drive-in furnace at about1100 degrees C. for 40 minutes to re-oxidize the surface boron-glasslayer created during the previous step to a thickness of about 0.5microns, as indicated by layer 188 of FIG. 2D.

Thereafter, as shown in FIG. 2D, a layer 190 of photoresist is depositedover the new oxide layer and patterned by removing all portions of thephotoresist in the entire transducer area 192, i.e., the area within thelarge rectangle 176 indicated by dashed lines in FIG. 3. Thereafter, thesilicon dioxide in the transducer area 192 is removed, thus leavingoxide layer portions 196 and 198 shown in FIG. 2E.

Next, as indicated in FIG. 2E, a short or shallow boron diffusion stepis performed in the open transducer area within rectangle 176, includingin the central region 200 between rim areas 182 and 184. The depth ofthis short boron diffusion is equal to the desired thickness for thesilicon diaphragm 136. For a thickness of 2 microns, for example, theshort boron diffusion step is performed at 1175 degrees Centrigrade for30 minutes.

Thereafter, if desired, the diaphragm area 136 may be covered with adielectric layer for protection against electrical shorts andenvironmental contamination. The dielectric layer 204 is preferably acompound layer comprised of a first sublayer 205 of thermally grownoxide 1000 angstroms thick and a second sublayer 206 of of CVD siliconnitride (Si₃ N₄) 1000 angstroms thick. The resulting composite layer 204is nearly neutral in stress (or in mild tension). By altering thethickness of its sublayers, such a composite layer 204 can be readilyadjusted to have a temperature coefficient closely matching that of theunderlining silicon diaphragm 136. The dielectric layer 204 is initiallygrown and deposited over the entire wafer 140, as indicated in FIG. 2F.Next, the dielectric layer 204 is removed over all areas, other than thediaphragm 136. This is done as follows. A layer 207 of photoresist isapplied and patterned using a fourth mask having a geometrycorresponding to the inside rectangle 178 shown in FIG. 3, so that apatterned photoresist portion 208 remains and covers the diaphragm area.Then, using a plasma (dry) etch of silicon nitride, followed by abuffered HF (wet) etch to remove the oxide layer, the dielectric layer204 is removed from all areas of wafer 140 other than the diaphragm 136,so that only dielectric layer portion 210 remains, as best seen in FIG.2G. The same buffered HF etch also removes the back side oxide layer144.

Thereafter, a bonding step is performed, as illustrated in FIG. 2G,wherein the silicon wafer 140 is batch bonded to the glass plate 32using an electrostatic (anodic) bonding technique. The alignment of thesilicon wafer 140 to the glass substrate 32 is straightforward since theglass is transparent. The bonding is preferably formed by heating theassembly 212 of the wafer 140 and glass substrate 32 to between 400 and450 degrees C., and then applying 400 to 600 volts DC from a suitablesource 214 across the glass plate 32 and silicon wafer 140 for twominutes. This forms a hermetic seal between the rim structure (e.g., rimportions 182 and 184) of the wafer 140 and the glass 32. Theelectrostatic bonding of a silicon wafer to glass is widely used toconstruct other types of silicon capacitive pressure transducers. Seefor example, Y. S. Lee and K. D. Wise, "A Batch-Fabricated SiliconCapacitive Pressure Transducer With Low Temperature Sensitivity", IEEETransactions on Electron Devices, Vol. ED-29, No. 1, pp. 42-48 (January1982), which is hereby incorporated by reference. Thus, theelectrostatic bonding step need not be further described here.

Following the bonding step, the silicon wafer glass assembly shown inFIG. 2G is immersed in an anistropic etchant for silicon (such asethylene-diamine/pyrocatechol/water (EDP)) and all of the silicon wafer140 is dissolved, except for the boron doped rim portions of the wafer,such as rim portions 182 and 184, and diaphragm 136 therebetween. Thus,only the boron-doped portions of the wafer 140 are retained, and theseportions constitute the patterned transducer chip 34 shown in FIG. 1.These portions do not etch in EDP, nor do the interconnect metals, theglass, or the oxide or other dielectric layers. The result of thisetching operation is a glass plate 32 containing a glass-silicontransducer 220 as shown in FIG. 2H.

FIG. 4 shows a portion of a glass substrate 138 having eight glasssubstrates 32 prior to being diced into individual substrates. The foursubstrates 32 in the center are shown with completed glass-silicontransducers 220 of the type shown in FIG. 2H. (To avoid cluttering theFigure, the glass substrates 32' are shown without their transducers220.) As can be readily appreciated by those skilled in the art, thefabrication process described with respect to FIGS. 1-3 is fullycompatible with automated batch processing techniques which will allowthe transducers 220 to be prepared en masse, that is by the hundreds (ormore) from a single silicon wafer. The glass substrates 32 shown in FIG.4 are diced along the vertical and horizontal dashed lines 222 and 224to separate them into individual substrates 32.

As should be evident from the foregoing, the transducer processdescribed above utilizes single-sided processing of silicon wafershaving normal thickness. It requires only three non-critical maskingsteps to produce the patterned silicon wafer diaphragm and rim structureor chip 34, and produces a very high yield. (If the optional dielectricSiO₂ --Si₃ N₄ layer is used, then a fourth mask is required.) The rimand diaphragm thicknesses are set by the deep and shallow borondiffusion steps with a precision of better than 0.1 microns, whilelateral dimensions are controlled by lithography to a precision ofbetter than 0.25 microns.

The rim size is scalable, but is typically 12 microns thick and 80microns wide. This is significantly smaller than those found inconventional pressure sensing structures, in which the diaphragm isformed from a back side anisotropic etch, and the width of the rim iscomparable to the wafer thickness (300 microns or more). Furthermore, inthe present approach, batch wafer bonding to the glass before waferdissolution eliminates handling of individual diaphragm structures untildie separation and final packaging. The glass 138 may be sawed intoindividual dies (i.e., glass plates 32) before or after the bonding ofthe chips 36 to the metal pads 60, 62, 106, 107 and 108.

FIG. 5 shows the measured pressure characteristics of the ultraminiaturesensor illustrated in FIG. 1. The reference cavity 135 for the sensor130 upon which the measurements were made was sealed and at atmosphericpressure. However, differential pressure measurement with an unsealedreference pressure inlet channel 134 is also possible with thetransducer 220. The sensor 30 upon which the measurements shown in FIG.4 were made had a diaphragm size of 260 microns×600 microns×2 microns, acapacitor plate separation of 2.2 microns, a zero-pressure capacitanceof 0.33 pF, a pressure range of about 500 mmHg and a pressuresensitivity of 1440 ppm/mmHg. By scaling lateral and vertical diaphragmdimensions, the pressure sensitivity of the transducer 220 can be scaledover several orders of magnitude.

The small size of the capacitance change associated with the sensor andthe need to simplify packaging by minimizing external leads poses achallenge to the design of the circuit chip. An oscillatory-type circuitwhich requires only two external leads has been developed in which thesupply current is pulse-period modulated by the applied pressure. Afunctional block diagram of this circuit is shown in FIG. 6. The circuit250 includes a pulse amplitude discriminator circuit 252, a two-stagecounter 254, and a Schmitt trigger oscillator circuit 256 including aSchmitt trigger oscillator 258, an enable switch 260, and a two-positionselector 262. Electrical power and signal communication is deliveredover conductors 266 and 268 which respectively are nominally at asolid-state circuit supply voltage VDD (such as +5 VDC) and groundpotential GND (0.0 volts). The operation of circuit 250 may be explainedin brief as follows. Schmitt trigger oscillator 258, in which the periodof oscillation is dominated by the charging time of the transducercapacitance C_(X), sets up an oscillation in the supply currentdelivered via conductors 266 and 268. The electrical pressure signaloriginating with capacitor C_(x) is then extracted by detecting thefrequency of current variations over the power lines 266 and 268.Tradeoff between pressure signal bandwidth and resolution can beattained by altering the length of the sampling time. Temperaturecompensation is accomplished by differencing the oscillation periodproduced using the transducer with that of an on-chip referencecapacitor C_(R) which may be a thin film capacitor integrally fabricatedwith the other circuit components on circuit 250 in IC chip 36. Thereference capacitor C_(R), together with the temperature coefficient ofthe circuit supplying power to capacitor C_(R), also serves as atransducer for on-chip temperature readout. Site andpressure/temperature transducer addressing is accomplished by signalingover the supply line 266 to circuit 252, which triggers an on-chipcounter 254 and allows one particular sensor on a bussed multisensorline to be activated, while inactivated oscillator circuits such ascircuit 256 are disabled. Thus multisite operation is possible. Bydepositing a thin film metal resistor having a high temperaturecoefficient of resistance on the diaphragm 130, it is possible tomeasure dynamic changes in temperature as well as pressure at eachsensing site to allow future thermal dilution measurements of bloodflow.

The addressable read-out circuit 250 in FIG. 5 may be fabricated on asingle integrated chip such as chip 36 shown in FIG. 1. Prototypes ofthe chip 36 have been fabricated using standard NMOS processing, withbeam leads to allow low-capacitance low-profile high-densityinterconnects to the transducer 220 and output leads 60, 62, 66 and 68via the glass substrate 32. Use of a hybrid arrangement, as shown inFIG. 1 where the IC chip 36 and pressure transducer 200 are separatelyfabricated and then connected together, has several importantadvantages: (1) the circuits 250 can be processed using standard ICfabrication techniques and may be realized using a chip foundry; (2) thecircuitry 250 is not exposed to the high voltage needed for theelectrostatic bonding process; and (3) working circuit chips 36 can beselected for bonding to transducers 220, thus improving yield.

FIG. 7 shows a detailed circuit diagram of the pulse amplitudediscriminator circuit 252. The circuit 252 contains sevenmetal-oxide-semiconductor (MOS) insulated-gate field effect transistors(FETs) 270 through 282. FETs 270-274 are enhancement-mode devices, whileFETs 276-282 are depletion-mode devices. The circuit 252 receives powerover supply lines 266 and 268, and receives two different types ofcommands over supply lines 266. The nominal voltage VDD is +5 volts DC.The first command signal is a clock signal which is delivered at +8volts DC. The second command signal is a reset signal which is deliveredat +11 volts DC. The first command signal is shown in FIG. 10 onwaveform 284. The pulses 286 are the clock pulses. The circuit 252produces three outputs signals, namely the RESET on line 290 and theMODE and MODE* signals on lines 292 and 294. A waveform 296 in FIG. 10shows the timing and voltage levels of the MODE signal. The MODE* signalis the complement of the MODE signal. (The asterisk symbol is used toindicate the complement of whatever signal it follows.) The operation ofcircuit 252 in FIG. 7 may be briefly explained as follows. When VDD line266 temporarily goes to 8 volts, it draws node 300 to a sufficientlyhigh level, causing gate 298 to transistor 272 to go high, which turnstransistor 272 on. Accordingly, MODE* goes from a high to a low logiclevel, and transistor 274 turns off, which makes the MODE signal on line292 switch from a low logic level to a high logic level. When VDD line266 returns to 5 volts, gate 298 returns to a low logic level causingtransistor 272 to turn off. Accordingly, the MODE* signal switches froma low to high logic level, and transistor 274 turns on, which causes theMODE signal on line 292 to switch from high to low. When the VDD linegoes to 11 volts, indicating a RESET command, it draws reset line 290from a low to high logic level.

FIG. 8 is a circuit diagram of the two-stage counter circuit 254 shownin FIG. 6. The circuit 254 includes a first stage 310 and a second stage312 interconnected by a single conductor 314. The output signal P online 316 of first stage 310 is the low-order bit output of counter 254,while the output signal CE (which stands for Chip Enable) is the outputfor the high-order bit of counter 254. The first and second stages 310and 312 each receive the MODE, MODE* and RESET signals respectively fromlines 292, 294 and 290. The circuit 254 provides as outputs the P and P*signals on lines 316 and 320 respectively from the first stage 310, andprovides as outputs the CE and CE* signals on lines 318 and 270 of thesecond stage 312. In the FIG 10 graph, waveforms 322, 324 and 326 showthe timing relationships for signals P, P', and CE respectively.

As shown in FIG. 8 the first stage 310 includes inverter 330, MOSFET332, NOR gate 334, inverter 336 and MOSFETs 338 and 339, all connectedas shown. The second stage 312 includes: inverter 340, MOSFETs 341 and342, NOR gate 344, inverter 346 and MOSFETs 348 and 349, all connectedas shown. The operation of stages 310 and 312 will now be brieflyexplained, and it will be assumed that the RESET line 290 remains at alow logic level, which causes NOR gates 334 and 344 to each function asa simple inverter. As long as the MODE* signal remains high, thetransistors 338 and 348 conduct. In stage 310, the NOR gate 334 andinverter 336 act as a latch to hold on the signal present on line 320whenever transistor 338 is on and transistor 332 is off. Similarly, instage 312, the NOR gate 344 and inverter 346 act as a latch whenevertransistor 348 is on (that is, conducting) and transistor 341 or 342 isoff. Returning now to first stage 310, assume line 314 (the P' signal)is low. When the mode signal arrives on line 292, transistor 332 turnson, causing line 350 to go low irrespective of its previous state, whichcauses line 316 to go high and line 320 to go low. While the MODE signalon line 292, the MODE* signal on line 294 is low and transistors 338 and339 are off. When the MODE signal on line 292 goes low, transistors 338and 339 turn on, thus latching in the low signal on line 320 andsimultaneously supplying a low signal on line 352 going to the input ofinverter 330. This causes the output of inverter 330 and line 314 to gohigh. However, since transistor 332 is now off, the high signal on line314 is unable to propagate through to line 350 at this time. When themode signal on line 292 goes high again, the high signal on line 314propagates through to line 350, thus causing line 316 to go high andline 320 to go low. When the mode signal goes low again, the low stateof line 320 is latched in via transistor 338. Thus, it will beappreciated that the output 316 and 320 of first stage 310 change stateeach time that the mode signal on line 292 goes high.

In contrast, the outputs 318 and 270 of the second stage 312 toggle,that is, change state, only with every second time that the MODE signalon line 292 goes high. This is because the stage 312 contains anadditional transistor 341 which only allows the output signal CE* online 270 to change state when the MODE signal on line 292 and the signalon line 314 from stage 310 are both high. In all other respects, theoperation of stage 312 is the same as stage 310. Note that when theRESET signal on line 290 goes high, the outputs of NOR gates 334 and 344are forced low irrespective of the state of input lines 350 and 356,thus causing both the P signal on line 316 and the CE signal on line 318to go low. Transistors 338 and 348 if on will latch in this RESET outputstate.

FIG. 9 shows a detailed block diagram of the trigger oscillator circuit256 shown in FIG. 6. The circuit 256 includes a MOSFET 360, which actsas the enable switch 260. In the circuit 256 shown in FIG. 9, the switch260 is turned on whenever the gate input line 270, containing the enablesignal CE* from the second stage 312 of the counter 254 is high. Inanother circuit 256' (not shown), the input to transistor 360 would bethe output signal CE on line 318 from the second stage 312 of counter254. Since the enable switch 260 must be on in order for the oscillatorcircuit 256 to operate, it will be appreciated that the input signal totransistor 360 is effectively an address signal which must be high inorder for the oscillator circuit 256 to be addressed. The Schmitttrigger oscillator 258 consists of three components, namely an invertingSchmitt trigger 362, inverter 364 and a source 366 of approximatelyconstant current which is provided at a predetermined level from supplyline 266 to output line 368. The current source 366 is set to producethe desired rate of charging of the transducer capacitor C_(X) and thereference capacitor C_(R) shown in FIG. 9. The switching circuit 262includes four enhancement-mode MOSFETs 370-376. The operation of circuit256 will now be briefly explained. When circuit is enabled by a highsignal on the input of transistor 360, and the output of inverter 364 online 378 is low, transistors 372 and 376 will be off, thus permittingcapacitors C_(X) and C_(R) to charge. When signal P on line 316 is high,transistor 370 is on, thus allowing transducer capacitor C_(X) tocharge, and the reference capacitor C_(R) will not charge since thecomplementary signal P* on line 320 will be low. Conversely, when thesignal P* on line 320 is high, transistor 374 will be on, thus allowingreference capacitor C_(R) to charge, while transistor 370 will be off,so that capacitor C_(X) cannot charge. The rate at which the capacitorC_(X) and C_(R) charge is determined by the rate at which current issupplied to line 368 from the constant current source 366. When eithercapacitor C_(X) or C_(R) charges to a predetermined threshold voltagelevel which exceeds the input threshold voltage required to turn Schmitttrigger 362 on, output of Schmitt trigger 362 on line 380 goes low,which causes the output of inverter 364 on line 378 to go high. Thisturns on transistors 372 and 376, discharging both capacitors C_(X) andC_(R). The voltage on line 368 immediately approaches zero volts, thusresetting the Schmitt trigger 362 and allowing the output of inverter364 to go low high, which turns off transistors 372 and 376. At thispoint, either capacitor C_(X) or C_(R) is allowed to begin chargingagain.

Waveform 382 in FIG. 10 illustrates the operation of the FIG. 9 circuitby showing the output voltage VOUT on line 368. As shown in FIG. 10, thetime period T_(X) between t1 and t2 represents the interval during whichthe transducer capacitor C_(X) is being repetitively charged anddischarged by circuit 256. Similarly, the time period T_(R) betweentimes t0 and t1 represents the interval of time during which the circuit256 is charging and discharging the reference capacitor C_(R). Duringthe time period T_(OFF) from times t2 to t4, the circuit 256 shown inFIG. 9 is disabled, thus allowing the voltage VOUT on line 368 toapproach the value of voltage VDD on line 266 as shown by waveformportion 384 of waveform 382 in FIG. 10. Since the constant source 366may be measured or otherwise calibrated at a known temperature, anychange in the rate of charging of reference capacitor C_(R) can becorrelated with reasonable accuracy to changes in temperature of theintegrated circuit chip 336 in which capacitor C_(R) is located. Giventhe small size of the sensor 30, and the proximity of capacitor C_(R) tothe diaphragm 130, will be appreciated that C_(R) provides a convenientand accurate method for determining the temperature of the pressuretransducer of sensor 30, so that the pressure readings obtained from thecharging time of transducer capacitor C_(X) can be accuratelycompensated for temperature by an external monitoring system whichexamines the charging rates of capacitors C_(X) and C_(R). Thesecharging rates are monitored by monitoring the frequency of the currentsignal drawn by circuit 252 over power supply lines 266 and 268. As iswell understood by those in the art, the amount of pressure applied tothe diaphragm 130 of pressure transducer 220 directly influences thecapacitance value of capacitor C_(X). For example, as pressureincreases, the capacitance value increases. Since changes in thecapacitance value of capacitor C_(X) results in proportional changes inthe charging time of capacitor C_(X), the pressure being applied to thediaphragm 130 can be readily determined by monitoring the frequency ofthe current signal on power lines 256 and 268 when capacitor C_(X) isallowed to charge by transistor 370 being on, and transistor 372 beingoff, as has been discussed with respect to FIG. 9.

As will be readily understood by those skilled in the art, the pressuretransducer 220 of sensor 30 can be operated in several ways. Forexample, the pressure transducer 220 can be sealed at ambient pressure,or under vacuum conditions. If sealed at ambient pressure, epoxy orother suitable sealing materials can be deposited at the openings ofboth reference channels 133 and 134 (see FIGS. 1 and 3). Onedisadvantage of sealing gas in the cavity or chamber 135 of the pressuretransducer 220 is that it results in a pressure transducer which has ahigh temperature coefficient on account of the pressure of the trappedgas naturally changing within sealed chamber 135 as the gas temperaturechanges. If the transducers 200 are to be sealed under vacuumconditions, this may be done en masse simultaneously while they arestill on the glass plate 138 before the glass plate sections 32 arediced into individual glass plates, by depositing a dielectric materialwith sealing properties directionally through a shadow mask at the mouthor opening of each of the reference channels 134 and 135. This task canbe carried out in a sputtering chamber using silicon dioxide (or thelike) as the sealing material using a shadow mask which only hasopenings above the reference channels.

Referring now to FIG. 11, an ultraminiature catheter system 400 isshown. The system includes a very small catheter 402 such as 0.5 mmouter diameter (OD) tubing made from polyethylene or other suitablematerial, which has a length sufficient for the medical (or other)purpose to which the catheter will be placed. The system shown has twopressure sensors 30a and 30n, which are preferably constructed likesensor 30 shown in FIG. 1, spaced apart by a predetermined distance 404such as 5 cm. The catheter 402 has three distinct sections: a cathetertip section 406, preferably round and gently pointed as shown tofacilitate insertion into and passage along the interior of smallerblood vessels; an intermediate section 40 between the two pressuresensors 30a and 30b; and an extension section 410 to provide a conduitthrough which the wire leads 266 and 268 from electronic pressure sensormonitoring equipment may pass to reach the first and closest sensor 30a.

Preferably, the catheter system uses only the two wire leads 266 and 268which are electrically connected in parallel to the two sensors 30a and30n, while physically being arranged in series. Prior to the assembly ofthe catheter system 400 the pressure sensors 30a and 30n, which eachinclude a glass plate 32 with the transducer chip 34 and read-outelectronics chip 36 mounted thereon, is partially encapsulated with abiomedically compatible material 414 (i.e., one that is non-toxic andwill not be adversely affected by the bodily fluids to which it beexposed) such as polyimide, silicone rubber, or the like, to seal offthe hollow cylindrical interiors of the catheter sections from bodilyfluids. The diaphragm 130 may have encapsulating material upon it,provided that the thickness of the layer of material upon the diaphragmis controlled so as to be at least about an order of magnitude moreflexible than the silicon diaphragm 310. The integrated circuit chip 36is preferably within the dry interior of the catheter section 408 (orsection 410) where it will not be contacted by bodily fluids.

The required catheter leads 266 and 268 are attached via soldering orthe like into the etched grooves 40, 42, 46 and 48 as has been explainedin FIG. 1. Finally, the ends of the glass plates 32 are inserted in thecatheter 402, leaving only the silicon diaphragms 130 exposed formeasurement.

Prototypes of the above-described multipoint pressure-sensing cathetersystem have been fabricated and successfully tested. Table I belowprovides typical specifications for our prototypes.

                  TABLE l                                                         ______________________________________                                        Catheter Size     0.5 mm OD                                                   Diaphragm Size    290 × 500 × 2 microns                           Transducer Die Size                                                                             0.45 × 1 mm                                           Circuit Die Size (Prototype)                                                                    0.45 × 1.1 mm                                         Zero-Pressure Capacitance                                                                       470 fF                                                      Pressure Accuracy 1 mmHg                                                      Pressure Range    500 mmHg                                                    Signal Bandwidth  50 Hz                                                       Temperature Compensation                                                                        Frequency Differencing                                                        Using an On-Chip                                                              Reference Capacitor                                         Power Supply      Single 5 V                                                  Signaling Levels  8 Volts - Addressing,                                                         11 Volts - Reset                                            Power Dissipation less than l0 mW                                             Output Signal     Small-Signal Supply Current                                                   Variation (600 A p-p Over                                                     850 microamps dc Baseline)                                  Number of Sensing Sites                                                                         2                                                           Number of Transducers/Site                                                                      2 (Pressure/On-Chip                                                           Temperature)                                                Number of External Leads                                                                        2                                                           ______________________________________                                    

It is recognized that those skilled in the art may make variousmodifications or additions to the preferred embodiments chosen toillustrate the invention without departing from the spirit and scope ofthe present contribution to the art. For example, the pressuretransducers of the present invention may be made much larger or smallerthan the embodiments described herein by appropriate scaling of variousdimensions. Also, the pressure transducer of the present invention maybe used to sense fluid flow and other conditions by providingappropriate means for causing deflection of the diaphragm are provided.Accordingly, it is to be understood that the protection sought and to beafforded hereby would be deemed to extend to the subject matter claimedand all equivalents thereof fairly within the scope of the invention.

We claim:
 1. An ultraminiature solid-state capacitive pressuretransducer which includes a parallel plate capacitor, comprising:anintegrally formed structure made from single-crystal material and havinga diaphragm and a rim extending about a substantial portion of theperiphery of the diaphragm, the structure having at least two dimensionsorthogonal to one another of less than one millimeter, and the diaphragmconstituting a plane supporting one plate of the parallel platecapacitor.
 2. A transducers as in claim 1, further comprising:a glassplate electrostatically bonded to at least part of the rim of thestructure.
 3. A transducer as in claim 1, wherein:the single-crystalmaterial is silicon, and the rim and diaphragm are heavily impregnatedwith at least first material alters an etching characteristic thereof incomparison to single-crystal material which contains substantially lessof such first material.
 4. A transducer as in claim 3, wherein:the firstmaterial is an impurity dopant selected from the group of dopantsincluding n-type materials and p-type materials.
 5. A transducer as inclaim 3, wherein the first material is boron.
 6. A transducer as inclaim 1, wherein the rim and diaphragm are made from a common wafer ofsingle-crystal material and the nominal width of the rim is at leastthree times smaller than the nominal thickness of the wafer from whichit is made.
 7. A transducer as in claim 6 wherein the nominal thicknessof the rim is no more than about 80 microns, and the nominal height ofthe rim is no more than about 12 microns.
 8. A transducer as in claim 1,wherein the nominal thickness of the single-crystal material of thediaphragm is no more than about two microns.
 9. A transducer as in claim1 having first and second electrically conductive parallel plates whichform the parallel plate capacitor, whose capacitance is adjustable by achange in pressure upon at least one of the plates, further comprisingan insulating nonflexible support substrate having an electricallyconductive plate formed on a first side thereof, which constitutes thefirst parallel plate, and wherein:the diaphragm of the integrally formedstructure constitutes the second parallel plate, the rim of theintegrally formed structure is bonded to the first side of the supportsubstrate and at least substantially surrounds the perimeter of thefirst parallel plate, the nominal thickness of the rim is no more thanabout 80 microns and the nominal height of the rim is no more than about12 microns, and the nominal gap between the first and second parallelplates is less than five microns.
 10. A transducer as in claim 9,wherein the overall size of the diaphragm measured in two orthogonaldirections along the plane of the diaphragm is no more than about fivehundred microns in each such direction.
 11. A transducer as in claim 1,having first and second electrically conductive parallel plates whichform the capacitor whose capacitance is adjustable by a change inpressure upon at least one of the plates, further comprising:a thincomposite dielectric layer formed on one side of the diaphragm; and aninsulating nonflexible support substrate having an electricallyconductive plate formed on a first side thereof, which constitutes thefirst parallel plate, and wherein the diaphragm of the integrally formedstructure constitutes the second parallel plate, the rim of theintegrally formed structure is bonded to the first side of the supportsubstrate and at least substantially surrounds the perimeter of thefirst parallel plate, the composite dielectric layer is on the side ofthe diaphragm facing the first parallel plate, and is formed of at leastfirst and second sublayers, the first sublayer including a thermallygrown oxide and the second sublayer including a deposited siliconnitride.
 12. A transducer as in claim 1, wherein the diaphragm of theintegrally formed structure is rectangularly shaped, is centrallylocated in and surrounded by the rim, and has an average thickness whichis at least a few times thinner than the average thickness of the rim.13. A solid-state capacitive pressure transducer including a parallelplate capacitor therein, comprising:an integrally formed structure madefrom single-crystal silicon semiconductor material and having adiaphragm and a rim that extends at least around a substantial portionof the periphery of the diaphragm, the diaphragm and rim being heavilyimpregnated with at least a first material which alters an etchingcharacteristic thereof in comparison to single-crystal material which isidentical to the heavily impregnated material of the diaphragm and rimexcept for containing substantially less of such first material, andwherein the diaphragm constitutes a supporting plane of one plate of theparallel plate capacitor.
 14. A transducer as in claim 13, furthercomprising:a glass plate electrostatically bonded to a generally flatsurface of the rim, such that the diaphragm is generally parallel to andspaced closely to but remains apart from the glass plate.
 15. Atransducer as in claim 13, wherein the first material is an impuritydopant selected from the group of dopants including n-type materials andp-type materials.
 16. A transducer as in claim 13, in which thetransducer is part of a pressure sensor, further comprising:aninsulating plate bonded on one side thereof to at least part of the rimof the integrally formed structure, a thin electrically conductive padlocated on the side of the insulating plate bonded to at least part ofthe rim, the conductive pad being spaced apart from and generallyparallel to the diaphragm and forming a stationary plate of thecapacitor of the transducer, with the diaphragm forming a flexiblesecond plate of the capacitor.
 17. A transducer including apressure-adjustable parallel plate capacitor, comprising:a firstelectrically conductive plate; and an integrally formed structure madefrom single-crystal silicon semiconductor material and having adiaphragm and a rim that extend entirely around the periphery of thediaphragm and mechanically supports the diaphragm in a nominal positionparallel to and at a predetermined nominal distance from the firstplate, the diaphragm and rim being electrically conductive and heavilyimpregnated with at least a first material which alters an etchingcharacteristic thereof in comparison to single-crystal material which isidentical to the heavily impregnated material of the diaphragm and rimexcept for containing substantially less of such first material, thediaphragm constituting a second electrically conductive plate, and beingsufficiently thin relative to its overall size to allow it to respond toa mechanical force applied which is thereto in the form of changingpressure in a range designed to be measured by said transducer, therebychanging the capacitance of the parallel place capacitor.
 18. Atransducer as in claim 17, wherein the rim and diaphragm are formed froma single wafer of single-crystal material and the nominal width of therim is at least three times smaller than the nominal thickness of thewafer from which it is made.
 19. A transducer as in claim 18 wherein thenominal thickness of the rim is no more than about 80 microns, and thenominal height of the rim is no more than about 12 microns.
 20. Atransducer as in claim 18, wherein the nominal thickness of thesingle-crystal material of the diaphragm is no more than about twomicrons.
 21. A transducer as in claim 18, wherein the nominalpredetermined distance between the first and second parallel plates isless than five microns.
 22. A transducer as in claim 18, furthercomprising:a support substrate having least one substantially flatsurface on a first side thereof, with the first electrically conductiveplate formed on the first side of the substrate, and a conductive traceformed on the first side of the substrate adjacent to and electricallyisolated from the first conductive plate, and wherein the rim extends atleast around a substantial portion of the periphery of the diaphragm andthe rim is in electrical contact with the conductive trace.
 23. Atransducer as in claim 22, wherein the support substrate:(1) isconfigured as an elongated rectangular slab, (2) is no more than aboutone millimeter long, and (3) is no more than about one-half millimeterwide.
 24. A transducer as in claim 22, wherein the support substrate isa glass material having a coefficient of thermal expansion closelymatched to that of single-crystal semiconductor material.